That's a good point! Because the compound noun rules in English normally call for all nouns, other than the head of the noun phrase, to be singular.

This applies to the singulars that denote single objects too: scissor lift, pant leg; not scissors lift or pants leg.

According to this principle, it should be datum point, where only the head noun point may be pluralized.

"Datum point" exists, but I've never seen or heard it in use; it's just a "confirm by Google".